Project Honeypot just published a report of their experience in processing 1 billion spam messages. Highlights for the impatient:
On the other hand "Fraud" spammers -- those committing phishing or so-called "419" advanced fee scams -- tend to send to and discard harvested addresses almost immediately. The increased average speed of spammers appears to be mostly attributable to the rise in spam as a vehicle for fraud rather than an increasing efficiency among traditional product spammers.
As an anecdote to reinforce this, on one site i administer, i set up a dedicated subdomain which was purely designed to catch spam. I placed some addresses in that domain on a web page, and within 1 day they had been harvested and 1 spam had been sent to each email address. No email to that subdomain has been seen since.
Check out Project Honeynet's full analysis.